Wire bonding is a commonly used technique for making interconnections between a semiconductor device and its packaging, or to connect a device to another device. For example, in one type of wire bonding called ball bonding, a wire is fed through a capillary of a wire bonding tool, and a charge is applied to the tip of the wire (using an electronic flame-off device, for example) to melt it and to form a ball of molten metal.
When bonding wires made of reactive materials such as copper or aluminum are used for wire bonding, there is a tendency for oxidation of the molten ball to occur when the melted material reacts with oxygen in the atmosphere. Oxidation of the molten ball degrades the quality of the subsequent ball bond that is formed. Therefore, it is usually necessary to provide a shielding gas comprising a relatively inert gas such as nitrogen or argon gas to occlude the wire during ball formation and prevent contact with oxygen. The shielding gas is generally supplied via a shielding gas implement having an outlet adjacent the capillary, for example via a slot or sleeve surrounding the capillary tip as shown in U.S. Pat. No. 7,628,307, the contents of which are incorporated by reference as if fully set forth herein.
It is important to ensure that the supply of shielding gas is stable, such that ball bonds are formed consistently and without surface oxides. If there are any leakages in the pipeline of the gas supply system feeding the shielding gas implement (or other gas implement employed by the wire bonding apparatus), this will significantly affect the stability of the gas feed.
Flow rate sensors can only measure the flow rate at their inputs. Accordingly, in order to be able to reliably measure leakage in a pipeline, two sensors are required: one at the input end, and one at the output end. A detected difference in flow rates between the inlet and outlet sensors indicates a leakage. A problem with attaching a sensor at the output end of a gas supply system of a wire bonding system is that such a sensor not only adds more cost, but also adds more weight to the system, which may not be allowed in some applications. In addition, many flow rate sensors have low resolution, and cannot sense variations in flow rate which are small but which might nonetheless have an effect on ball formation.
There remains a need for a system and method for reliably detecting leakages in shielding gas supply systems for wire bonding.